Biostar M6VCF mobo and RAM

I have an old Biostar M6VCF mobo with a Celly 1GHZ and 2x128MB PC100 SDRAM installed in slots 1 and 2. I have taken another PC100 128MB stick from a working computer, and put it in slot 3. The computer doesn't recognize the RAM as being installed, and occasionally will only recognize 128 of the 384MB.

Any ideas on why this is? Is it standard for mobo to only have 2 slots that work? I tried RAM in all different combinations on the mobo, but Slot 1 & 2 are the only ones that work. Maybe Slot 3 is dead??

Spec:
Intel P4 2.4B
MSI 645E Max-U Mobo
1GB DDR333
R9600XT 128MB
SB Live 5.1
WD 60GB
Maxtor 120GB
LG DVD+-R/RW
WIN2K PRO SP4